The present invention relates to brushless direct current electric motors and particularly to small electric motors that are used in various tools to make measurements in hostile environments at elevated temperatures. For example, in a copending application Ser. No. 516,589, filed Apr. 30, 1990, there is disclosed a tool which is adapted to make measurements in thermal injection wells. The term `thermal injection wells` is used to refer to wells which are used to inject steam or other heated fluids into hydrocarbon-containing formations to reduce the viscosity of the hydrocarbon and increase the production. The injected fluids can be in the range of 300.degree.-500.degree. F. and contain caustic elements. Thus, measuring instruments disposed in a thermal injection wells must operate at elevated temperatures and in a hostile environment.
In the above referenced copending patent application, there is disclosed a tool designed to measure both the quality and quantity of steam being injected into particular locations in a thermal injection well. The tool utilizes a brushless direct current electric motor which is used both as a motor to drive elements of the tool and as a generator to measure current that is produced. The tool requires means for measuring the speed of the motor or the generator as well as the stall torque and current flow to the motor. This necessitates the use of direct current motors.
The prior art direct current motors available all rely upon wound armatures that require a commutator and brushes for their operation. The use of brushes can cause maintenance problems, produce unwanted electrical noise in the downhole measuring tool, and introduce large frictional drag limiting the low-end torque measurement. While permanent magnet direct current motors are known and would eliminate the need for brushes, none are available that will withstand the temperature conditions existing in thermal injection wells. One of the major problems with prior art direct current motors using permanent magnets for the rotor is the mounting of the magnets on the motor shaft so that they remain fixed in position at all times, even at elevated temperatures. Simple clamping means cannot be used since the clamps tend to distort the magnets and loosen at elevated temperatures. Various mechanical means have been used to fasten the permanent magnets onto the motor shaft. Mechanical means require machining of the magnets which distorts the magnetic field of the magnets. Use of solder or other bonding agents causes large stresses in the magnet which can lead to cracking, especially when coupled with thermal stresses on entering higher temperature environments.